You want to cover as much area as you can, while spending the least amount of money on wards. For example, if you are on blue side, the mid laner can ward the enemy wraiths, while the top laner can ward the "corner" between baron and the tribush top, so you can see the top laner/the jungler come towards you long before they actually get there. Then, if you ward the entrance to dragon, you can see bottom going to mid/ the enemy jungler heading from blue buff to either mid or bot. If you're agressive, you can plant wards deeper into the jungle, such as by blue or red buff, so you can see when it is up to take it, and so you can see enemies coming to you if you pushed the first tower, since it'll be easier for you to get ganked.